This application is based on French Patent Application No. 01 12 059 filed Sep. 18, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the invention
The present invention concerns a heat transfer device, in particular a heat transfer device suitable for evacuating the dissipated heat generated by onboard equipment on board a satellite.
2. Description of the prior art
The equipment on board an operational satellite in orbit dissipates a great deal of heat. It is therefore necessary to provide a heat transfer device for evacuating heat to prevent the satellite from overheating by transferring heat from the interior of the satellite to space.
A number of prior art heat transfer systems include one or more deployable radiators connected to the satellite and provided with a heat transfer device. The heat transfer device is always based on the use of a fluid flowing between a hot area, i.e. an area in which heat is dissipated, and a cold area, i.e. an area in which the heat absorbed by the fluid is transferred to the external environment. The operating principle of these devices is therefore based on the evaporation/condensation properties of the fluid used.
A first type of prior art heat transfer device is the heat pipe. This system includes a metal (for example aluminum) tube in which flows a heat exchange fluid (generally ammonia) and uses the properties of two-phase liquid-vapor flow and the capillary properties of liquids. Thus a heat pipe is a two-phase closed system in which vapor created in the hot area (evaporation area) is aspirated toward the cold area (where the pressure is lower), where it condenses on the metal wall of the tube. The liquid phase of the fluid used travels along the metal wall of the tube in the opposite direction to that in which the vapor phase of the fluid, which remains confined to the center of the tube, flows. This return of the fluid along the wall is achieved by a capillary structure (wick or longitudinal grooves) connecting the two ends of the tube and which serves both as a capillary pump and as a separator of the two liquid-vapor phases.
Heat transfer devices using heat pipes, although widely used in satellites, nevertheless give rise to a number of problems.
First of all, their performance in terms of heat transport capacity is limited to a few hundred W.m. Accordingly, these heat transfer devices are inadequate for high-power telecommunication satellites and are ill-suited to the distances and the heat paths between the hot and cold sources.
Furthermore, for thermal tests to be carried out on the ground, it is necessary to ensure that the heat pipes are horizontal or to have the evaporation areas below the condensation areas, as otherwise the liquid must rise by capillary action, against the force of gravity.
Accordingly, the use of single-phase or two-phase fluid-loop heat transfer devices has been preferred.
Single-phase fluid-loop heat transfer devices operate in accordance with a principle similar to that of central heating, using the sensible heat of the fluid, and therefore with high temperature variations. The fluid used in the heat transfer device (freon, water, ammonia, etc) absorbs the heat dissipated by the equipment, and its temperature therefore rises, and rejects that heat when it is cooled in one or more radiators.
Although capable of absorbing significantly more heat than is possible using heat pipes, this type of heat transfer device is nevertheless unsatisfactory, in particular for high-power satellites.
In effect, they are active devices and require mechanical pumping using an electrically powered pump, which must produce a high fluid flowrate, because of the transfer principle employed. They therefore consume too much pumping power for effective heat control.
What is more, a mechanical pump gives rise to problems of vibration, maintenance and service life.
Accordingly, at present it is preferred to use two-phase capillary-pumped fluid loops using, like heat pipes, the latent heat of evaporation of the fluid to absorb and reject heat. The heat exchange fluid then changes state when it flows in the loop. It evaporates on absorbing heat dissipated by the equipment in the evaporator and condenses, rejecting the heat into one or more condensers on the radiator. The fluid is circulated by a capillary pump in the evaporator. The vapor and liquid phases are separated, except in the condenser where they flow in the same direction, in contrast to the heat pipe, in which the two phases flow in opposite directions in the same tube.
In terms of heat transfer capacity, this type of heat transfer device is significantly more effective than heat pipes for a much more limited capillary structure (only the evaporator has this pumping structure).
However, there are still problems for high-power satellites such as modern telecommunication satellites.
In effect, given the powers to be dissipated within such satellites, large surface areas are required on the deployable radiators. These surface areas can no longer be obtained from a radiator with only one panel, whose surface area can only with difficulty exceed a few m2, whereas an additional global radiating surface area of 60 m2 is needed.
Accordingly, the deployable radiators must comprise a plurality of mechanically interconnected panels.
The drive pressure available in two-phase capillary-pumped fluid loops limits the heat transfer distance for high powers. Accordingly, the same two-phase capillary-pumped fluid loop cannot be used from one end to the other of the panels of a large deployable radiator.
The object of the present invention is therefore to provide a heat transfer device for use on board a high-power satellite and in particular for use in conjunction with deployable radiators of said satellite including a plurality of panels, without significant limitation of its heat transfer capacity.
To this end the present invention proposes a heat transfer device including a capillary-pumped first fluid loop including an evaporator situated on a satellite in the vicinity of a source of dissipated heat and a condenser connected by heat transfer means to the evaporator and situated on a deployable radiator panel of the satellite, in which heat transfer device the deployable radiator comprises at least two panels, the heat transfer device itself further comprises at least one second capillary-pumped fluid loop, the fluid loops are connected in cascade with each other so that the evaporator of each fluid loop other than the first fluid loop is on the same panel as the condenser of the preceding loop and the condenser of each fluid loop other than the first fluid loop is on the panel next to that carrying the condenser of the first loop, and the evaporator of one loop is connected to the condenser of the same loop by flexible heat transfer means.
Thus, according to the invention, a plurality of cascaded loops is used instead of using a single capillary-pumped fluid loop over the whole of the surface area of the panels of the deployable radiator.
This provides a reliable, passive and modular system for evacuating the heat dissipated by high-power satellites in particular. The flexible heat transfer means enable the deployable radiator to be folded up, in particular before injecting the satellite into its orbit.
In an advantageous embodiment of the invention, heat is transferred between the condenser of one loop and the evaporator of the next loop by means of at least one heat pipe. This assures efficient transfer from one loop to the other.
Also, the evaporator of one or more loops can be formed of a plurality of individual evaporator circuits connected in parallel or in series with each other.
Similarly, the condenser of one or more loops can be formed of a plurality of individual condenser circuits connected in parallel or in series with each other.
Other features and advantages of the present invention will become apparent on reading the following description of one embodiment of the invention, which is given by way of illustrative and non-limiting example.